1. Field of the Invention
The present invention relates to micromachined devices and, more particularly, to acceleration protection for micromachined devices.
2. Description of Related Art
Precision-guided or xe2x80x9csmartxe2x80x9d projectiles have become the weapons of choice in modern warfare due to their high accuracy and the reduction in casualties of both friendly forces and civilians that may be achieved through their use. Currently, highly precise guided munitions that have been tested and proven ready for combat include missiles, such as the U.S. Navy""s Tomahawk Land-Attack Missile (TLAM) and the U.S. Air Force""s Conventional Air-Launched Cruise Missile (CALCM), and laser, GPS, or infrared guided smart bombs. However, due to their high cost (about $1 million for each TLAM and about $2 million for each CALCM), and due to the complex systems (e.g., aircraft, aircraft carriers, submarines, etc.) required to deliver missiles and smart bombs, the use of precision-guided munitions is not always practical for all military targets. For example, there are situations where the use of short time-of-flight, close-range, high fire-rate, and relatively inexpensive artillery projectiles would be preferred over the use of a single TLAM, CALCM, or smart bomb.
Recent testing has shown that the precision guidance of spinning, gun-fired projectiles is possible: the Office of Naval Research""s xe2x80x9cCompetent Munitions-Advanced Technology Demonstration . . . resulted in the successful test firing of a 5xe2x80x9d projectile with a GPS/MEMS INS (Global Positioning System/MicroElectroMechanical System Inertial Navigation System) installed in the fuse section. The GPS/MEMS INS was used to steer the projectile toward a target using a set of movable, nose-mounted fins.
MEMS devices may be manufactured on a large scale using photolithographic techniques to etch silicon wafers, in much the same way that traditional microelectronic integrated circuits are produced in the electronics industry. In silicon-based MEMS devices fabricated using conventional integrated circuit techniques, three-dimensional structures can be integrated with electronic circuitry on the same chip, offering great potential for improvements of sensors, actuators, and other devices. Initially, MEMS devices were strictly silicon-based, like microelectronic devices, but today the term represents complete miniature devices that may or may not be silicon-based, and that can be produced using methods other than photolithographic techniques.
MEMS devices and sensors that may be used in guidance systems can include gyroscopes and accelerometers. Gyroscopes, accelerometers, and other devices may have one or more proof masses that may be suspended above a substrate by spring elements mounted to the substrate. The proof mass is generally made to oscillate at a precise frequency axially and parallel to the substrate by an electronic drive mechanism. As used herein, the term xe2x80x9cproof massxe2x80x9d is defined broadly to include any mass suitable for use in a MEMS system. Proof masses typically also include electrical sense elements interleaved with complementary elements on or attached to a sense plate.
MEMS gyroscopes and accelerometers meet at least one criterion for use in gun-launched projectiles: they are extremely small compared to conventional instruments (the system used in the Competent Munitions Demonstration occupied a volume of 13 cubic inches). Because they do have moving parts, however, MEMS devices are still susceptible to failure due to extreme vibration and acceleration. Proof mass suspension elements, drive elements, and sense elements in gun-launched projectiles could easily be damaged due to forces that greatly exceed those forces a sensor is designed to measure; MEMS sensors that are used to guide gun-fired projectiles must withstand more than 10,000 Gs of launch acceleration, as well as vibrations in virtually all directions that are encountered when projectiles spin, xe2x80x9cchatterxe2x80x9d or ballot in the gun barrel, and when they leave the gun barrel.
Thus, preventing damage to the moving parts of MEMS sensors and other devices while still allowing normal operation is essential.
In a first principal aspect, a method and apparatus for preventing damage to a MEMS device during a period of high acceleration is disclosed. The MEMS device may have a proof mass mounted to a substrate using spring elements. The proof mass may have a normal range of motion. The method includes applying a DC voltage between the proof mass and a non-suspended structure of the device, such as a sense plate mounted on the substrate. The applied DC voltage is sufficient to create an electrostatic force that moves the proof mass to a position outside the normal range of motion.
In a second principal aspect, a micromachined device that includes a proof mass flexibly suspended above a substrate is disclosed. The proof mass has a normal operating range of motion toward and away from the substrate. The device can also include one or more non-suspended structures mounted on the substrate, and an insulating layer positioned between the proof mass and the substrate. The insulating layer prevents electrical contact between the proof mass and the at least one non-suspended structure when a DC voltage is applied between the proof mass and the non-suspended structure. The DC voltage causes the proof mass to move toward the substrate beyond the normal operating range of motion; the insulating layer limits the motion and prevents electrical contact between the proof mass and the non-suspended structure.
These as well as other aspects and advantages of the present invention will become apparent to those of ordinary skill in the art by reading the following detailed description, with appropriate reference to the accompanying drawings.